This invention pertains to variable-speed electrical machines and more particularly to electrical machines having a frequency dependent feedback current controlling the voltage and speed characteristics.
It is common practice for machines with electrical excitation that the exciter current is controlled by the rpm using mechanical or electronic controllers with respect to obtaining the required pattern of voltage or output. For machines with permanent magnetic excitation it is also known to have a magnetic flux running as a shunt circuit to the working flux. The size of this shunt current is variable using a controller with mechanical or electrical means which thereby provides indirect control of the useful current. The controllers specified are often expensive and very susceptible to failure.
In electrically driven vehicles in which a d.c. current generator is installed in the vehicle and driven by an internal combustion engine providing current to the drive motors (for example, a diesel-electric motor coach), known designs have incorporated the principle of operating the generator unsaturated with self-excitation and constant torque in order to attain near independence of the drive input from the relevant drive impedance, which even with very low rpm variations brings about very marked voltage variations and thus produces a practically constant current output. While this system avoids output controllers, the generators, due to the unsaturated operation, are inadequately utilized and further the danger exists that damaging overvoltages will occur.
It is an object of this invention to provide an electrical machine having a desired characteristic of induced voltage as a function of the rpm without a controller. It is a further object of this invention to provide an electric machine that utilizes the electrical capabilities of the machine.